A separation method using a membrane has lots of advantages over the method based on heating or phase-changing. Among the advantages is high reliability of water treatment since the water purity required can be easily and stably satisfied by adjusting the size of the pores of a membrane. Furthermore, since the separation method using a membrane does not require a heating process, a membrane can be used with microorganism which is useful for separation process but may be adversely affected by heat.
The membrane employing separation methods may include a method using a hollow fiber membrane module which comprises a bundle of hollow fiber membranes; and a method using a flat sheet membrane module. Conventionally, the membrane module has been widely used in a micro-filtration field for producing axenic water, drinking water, super pure water, and so on. Recently, however, the application of the hollow fiber membrane module is being expanded to include sewage and waste water treatment, solid-liquid separation in a septic tank, removal of suspended solid(SS) from industrial wastewater, filtration of river, filtration of industrial water, and filtration of swimming pool water.
One kind of the membrane modules is a submerged-type membrane module which is submerged into a water bath filled with fluid to be treated. Negative pressure is applied to the inside of the membranes, whereby only fluid passes through the wall of each membrane and solid elements such as impurities and sludge are rejected and accumulate in the water bath. Generally, the plural submerged-type membrane modules are used by the unit of cassette installed in a frame.
In more detail, as shown in FIG. 1, feed water to be treated is stored in a water tank 20, and is supplied to a water bath 10 where plural membrane cassettes 11 are submerged thereinto. As a constant negative pressure is applied to the plural membrane cassettes 11 by a suction pump 30, only fluid passes through the wall of each membrane in the membrane cassettes 11, and solid elements such as impurities and sludge are rejected. Fluid passing through the membranes 11 (hereinafter, referred to as “filtered water”) is provided to a filtered-water tank (not shown).
The plural membrane cassettes 11 are provided in the water bath 100 while being maintained at sufficient intervals, whereby the feed water supplied to the inside of the water bath 100 passes evenly through the respective membrane cassettes 11. Thus, when a water treatment process reaches an equilibrium condition, the respective membrane cassettes 11 treat the feed water of the similar impurity concentration.
The respective membrane cassettes 11 has about 90% recovery rate, wherein the recovery rate indicates the quantity of filtered water produced by the membrane cassettes 11 with respect to the total quantity of feed water supplied to the membrane cassettes 11. Thus, the filtering system shown in FIG. 1 has only about 90% recovery rate. That is, even though residue discharged out of the water bath 10 after being treated by the plural membrane cassettes 11 is in a slurry type containing a large amount of solid elements, a considerable amount of untreated water still remains in the slurry-type residue.
As a result, there is a need for an additional concentration apparatus 400 so as to recover the water which has not been collected by the plural membrane cassettes 11. This concentration apparatus 400 generally uses a fabric filter or a rough filtering method such as a sandblast method. In this reason, the water recovered by the concentration apparatus 400 can not be regarded as being the same as the water recovered by the membrane cassettes 11. Thus, the water additionally recovered by the concentration apparatus 40 is provided to the water tank 20, which can not be incorporated into the calculation for the total recovery rate of the filtering system.
In order to accomplish the total recovery rate of 96% or more, the filtering system shown in FIG. 1 requires more filtration capacity corresponding to the additional recovery rate, and requires the additional concentration apparatus 40, which is disadvantageous in that the filtering system becomes complicated and uneconomical.
Another method for raising the total recovery rate is to provide a 2-stage filtering system including two water baths instead of the aforementioned concentration apparatus 40. That is, in addition to the main water bath 10, there is the supplementary water bath for additionally recovering the water from the residue discharged out of the main water bath 10 by using the membrane cassettes submerged into the supplementary water bath. This 2-stage filtering system is advantageous in that the first filtering process using the main water bath enables to recover the treated water amounting to about 90% of the total feed water, and the following second filtering process using the supplementary water bath enables to additionally recover the treated water amounting to about 6% of the total feed water. However, this 2-stage filtering system is also problematic because it requires the additional structure for the second filtering process.